Computer restarts for no reason

We have a user who’s Dell Dimension machine running Win98 keeps restarting for no reason. It is hooked up to an APC UPS. I ran Dell diagnostics and it showed the hardware components to be in working condition. I decided to disconnect it from the UPS and run it on a surge protector. THe past 5 days it has been fine with no restarts.

THis morning when the user came in the machine diplayed a blue screen error that stated the following: Warning! System memory is running very low. NAV auto-protect may not be able to function properly.

Any ideas? Is the processor, power supply, motherboard, or perhaps memory module? Any help is appreciated.